Working with Markers

General Information

"Metadata" refers to extra information that is stored with an image, but is not necessary for decoding the image. It can contain information about the camera used to take the image, the author, date and time the image was taken, where the image was taken, whether the camera has GPS capabilities, etc. When an image is modified, you should preserve the original information, if possible. Metadata includes markers, tags and comments. This topic will deal primarily with markers. For more information on tags and comments, refer to the following:

Implementing TIFF Comments and Tags

Types of File Comments

JPEG files contain metadata information in user-defined markers. The user-defined markers are in the range 0xE0 to 0xFE. These markers can have a maximum size of 0xFFFD, or 65533 bytes. The markers below 0xE0 are reserved for the encoding of the image and inserting a reserved marker can make a file invalid. Therefore, you should not use reserved markers unless you are well acquainted with markers and image encoding in JPEG files.

TIFF files do not contain markers, however, LEADTOOLS provides a "workaround" for transferring Exif comments from JPEG files to TIFF files, and vice versa. The Exif metadata information from a TIFF file will be loaded as an APP1 marker. Therefore, you can load Exif metadata from an uncompressed file and store it in a compressed Exif file. You can also take metadata information from a compressed Exif file and store it in an uncompressed Exif file. Please note however, that some information cannot be stored inside uncompressed Exif files. For example, audio data is stored in APP2 markers and there is no built-in support for saving APP2 markers in TIFF files. However, you can still save the audio data yourself, using a custom tag. For more information on saving custom tags, refer to Implementing TIFF Comments and Tags.

Handling Markers

LEADTOOLS provides several functions for loading, creating, setting, modifying, getting information from and freeing markers.

To load markers from an existing JPEG file, call L_LoadMarkers. Although TIFF files do not support markers, LEADTOOLS provides limited support through the use of APP1 Exif markers. When you call L_LoadMarkers for a TIFF file, an APP1 marker is created and all the TIFF, GPS and Exif comments will be stored in it. In addition, any other tag with an ID greater than 0x8000 (32768) will be stored in the APP1 marker. The other information will be considered as being useful only for the image and will not be loaded into the APP1 marker. The only exception is the resolution information, which will be loaded in the APP1 marker.

The L_CreateMarkers function lets you create a collection of markers from scratch. You can then insert markers with L_InsertMarker. After you call L_InsertMarker once, you can call L_EnumMarkers and insert markers using the LEADMARKERCALLBACK callback.

Markers, like tags and comments can be set so that when a file is saved using the L_SaveXXX functions, the markers, tags and/or comments that have been set will be saved too. Set the markers to save using the L_SetMarkers function. Once markers have been set using L_SetMarkers, all the save operations from the current thread will use these markers. If you do not want to save any markers, call L_SetMarkers with the hMarkers parameter set to NULL. To get the markers that were set by the last call to L_SetMarkers, call L_GetMarkers.

Please note that both the L_SetMarkers and L_GetMarkers functions use copies of the list of markers. Therefore, the resources associated with these lists of markers should be freed by calling L_FreeMarkers when the markers are no longer needed.

Markers set using L_SetMarkers can also be written to files using the L_WriteFileComment and L_WriteFileTag functions. The comments or tags being written by these two functions will overwrite any similar comments or tags contained in the markers. The L_WriteFileMetaData function can also be used to write markers, tags and comments to a file. To summarize:

Markers are generally modified by deleting all markers of a certain type. To delete all markers of a certain type, use the L_DeleteMarker function. Currently there is no function for deleting a marker with a certain index because markers are not to be manipulated like an array. When you have more than one marker of the same type, it is because the markers are storing more than 64K of data. For example, if you have multiple APP2 markers in a collection, deleting just one of those APP2 markers will invalidate all the APP2 markers in that collection. If a marker MUST be deleted using it index, call L_EnumMarkers and delete the specific marker or create a new marker collection using L_CreateMarkers and insert all markers except the marker to be deleted.

The L_EnumMarkers function can be used to enumerate and modify a marker collection. It is a powerful function, used internally by most marker manipulation functions. For each marker enumerated by L_EnumMarkers, an ENUMMARKERSCALLBACK function is called. This callback function lets you delete markers from or insert markers to the marker collection being enumerated. This allows you to modify the marker collection passed to the L_EnumMarkers function. To insert a new marker in a marker collection, use L_InsertMarker. You can get a marker by index using the L_GetMarker function.

To determine how many markers are in a collection, call L_GetMarkerCount.

Finally, to create a copy of a marker collection and obtain a handle to the new marker collection, use L_CopyMarkers.